Power electronics apparatuses such as a switching power supply, an inverter and a lighting circuit of a lighting fixture incorporate a power transformer circuit for transforming power. The power transformer circuit incorporates a switching circuit for transforming a direct current to an alternating current having rectangular waves. Consequently, the power transformer circuit develops a ripple voltage having a frequency equal to the switching frequency of the switching circuit, and noise resulting from the switching operation of the switching circuit. Such a ripple voltage and noise affect other apparatuses. It is therefore required to provide a means for reducing the ripple voltage and noise between the power transformer circuit and the other apparatuses or lines.
LC filters, that is, filters each incorporating an inductance element (an inductor) and a capacitor, are often used as a means for reducing a ripple voltage and noise. The LC filters include a T filter and a π filter, in addition to the one incorporating an inductance element and a capacitor. A typical noise filter for suppressing electromagnetic interference (EMI) is a type of LC filters, too. A typical EMI filter is made up of a combination of discrete elements such as a common mode choke coil, a normal mode choke coil, an X capacitor, and a Y capacitor.
Recently, power-line communications have been developed as a potential communications technique used for creating communications networks in homes. Through the power-line communications, high-frequency signals are superimposed on a power line to perform communications. When the power-line communications are performed, noise emerges on the power line because of the operations of various electric and electronic apparatuses connected to the power line, which causes a reduction in quality of communications, such as an increase in error rate. It is therefore required to provide a means for reducing noise on the power line. Moreover, it is required for the power-line communications to prevent communications signals on an indoor power line from leaking to an outdoor power line. The LC filters are used as a means for reducing noise on the power line and for preventing communications signals on the indoor power line from leaking to the outdoor power line as thus described, too.
There are two types of noise propagating along two conductor lines: one is normal mode noise that creates a potential difference between the two conductor lines, while the other is common mode noise that propagates along the two conductor lines with identical phases.
In many cases there exist both normal mode noise and common mode noise on an actual conductor line although the proportions thereof vary. Noise suppressing circuits capable of reducing both normal mode noise and common mode noise are therefore desired.
The Published Unexamined Japanese Patent Application Heisei 8-213242 (1996) discloses a choke coil for reducing normal mode noise and common mode noise. The choke coil comprises a pair of windings, and first and second magnetic cores around which the pair of windings are wound. The first magnetic core is made of a low-permeability material while the second magnetic core is made of a high-permeability material. A specific gap is created between the first and second magnetic cores. In this choke coil, when a current of common mode is fed through the pair of windings, a magnetic flux produced by the windings attenuates in the second magnetic core, and common mode noise is thereby reduced. When a current of normal mode is fed through the pair of windings in the choke coil, a magnetic flux produced by the windings attenuates in the first magnetic core, and normal mode noise is thereby reduced.
The Published Unexamined Japanese Patent Application Heisei 9-46020 (1997) discloses a noise filter for reducing normal mode noise and common mode noise. The noise filter comprises a coil, an X capacitor and a Y capacitor all of which are combined.
The Published Unexamined Japanese Patent Application Heisei 2-206360 (1990) discloses a power supply circuit comprising a filter circuit for normal mode noise and a filter circuit for common mode noise. The filter circuit for normal mode noise incorporates two coils inserted to two conductor lines, respectively, and an X capacitor connecting the two coils to each other. The filter circuit for common mode noise incorporates two coils inserted to two conductor lines, respectively, and a Y capacitor connected to the two coils.
The Published Unexamined Japanese Patent Application Heisei 9-102723 (1997) discloses a line filter using a transformer. The line filter comprises the transformer and a filter circuit. The transformer incorporates a secondary winding inserted to one of two conductor lines for transmitting power from an alternating power supply to a load. The filter circuit has two inputs connected to ends of the alternating power supply, and two outputs connected to ends of a primary winding of the transformer. In the line filter, the filter circuit extracts noise components from the supply voltage and supplies the noise components to the primary winding of the transformer, so that the noise components are subtracted from the supply voltage on the conductor line to which the secondary winding of the transformer is inserted.
The conventional LC filters have a problem that, since the filters have a specific resonant frequency determined by the inductance and the capacitance, a desired amount of attenuation is obtained only within a narrow frequency range.
It is required for a filter inserted to a conductor line for power transfer that a desired characteristic be obtained while a current for power transfer flows and that a measure be taken against an increase in temperature. Therefore, a ferrite core having a gap is typically employed as a magnetic core in an inductance element of a filter for a power transformer circuit. However, such an inductance element has a problem that the characteristic thereof becomes close to the characteristic of an air-core inductance element, so that the inductance element is increased in size to implement a desired characteristic.
A typical EMI filter incorporates a filter for reducing normal mode noise and a filter for reducing common mode noise. As a result, the EMI filter has a problem similar to that of the above-mentioned LC filter and has a problem that the number of components is increased and the EMI filter is thereby increased in dimensions.
The choke coil disclosed in the Published Unexamined Japanese Patent Application Heisei 8-213242 is designed such that normal mode noise and common mode noise are reduced only by the cores and the windings. Therefore, it is difficult to effectively suppress common mode noise and normal mode noise in a wide range of frequencies in this choke coil.
The noise filter disclosed in the Published Unexamined Japanese Patent Application Heisei 9-46020 is designed such that normal mode noise is reduced only by the X capacitor while common mode noise is reduced only by the Y capacitor and the coil. Therefore, it is difficult to effectively suppress common mode noise and normal mode noise in a wide range of frequencies in this noise filter.
The filter circuit disclosed in the Published Unexamined Japanese Patent Application Heisei 2-206360 comprises the filter circuit for normal mode noise and the filter circuit for common mode noise. As a result, the filter circuit has a problem that the number of components is increased and the filter circuit is thereby increased in dimensions, like the above-mentioned typical EMI filter.
The line filter disclosed in the Published Unexamined Japanese Patent Application Heisei 9-102723 is capable of reducing normal mode noise but not capable of reducing common mode noise.